1. Field of the Invention
The present invention relates generally to multifocal lenses, and more particularly to multifocal lenses with powers which are intrinsically both diffractive and refractive powers. The invention relates even more particularly to multifocal lenses which provide simultaneous refractive and diffractive powers without exhibiting optical steps on a lens surface, common with diffractive lenses. The invention also relates to multifocal lenses in which at least two powers can be attributed to arbitrary relative intensities completely independent of one another.
2. Description of the Prior Art
A diffractive lens generally consists of any number of annular lens zones of equal area; such zones are usually called Fresnel zones. Between adjacent zones optical steps are provided with associated path length differences t which usually are absolutely smaller than a design wavelength xcex. The area or size of the zones determines the separation between the diffractive powers of the lens; this separation increases with decreasing zone area. The optical path length difference t determines the relative peak intensities of the various diffractive powers, e.g. for t=xcex/2 there are two principal diffractive powers, the 0-th and the 1-st order diffractive power, respectively, and both exhibit a peak intensity of (2/xcfx80)2=40.5%, where 100% is the peak intensity of a lens with identical Fresnel zones but with zero path length differences between any and all zones. The latter lens is a xe2x80x9cnormalxe2x80x9d refractive lens. For absolute path length differences smaller than half the design wavelength, the zeroth order power is dominant, for xcex greater than abs(t) greater than xcex/2 the first diffractive order power carries the maximum relative intensity.
It is of paramount importance to note that with any single Fresnel lens zone of a diffractive lens, a refractive power is associated; this refractive power can be calculated by refracting an incoming light ray using Snell""s refraction law. The Fresnel zone may exhibit a uniform refractive power, but it can also exhibit a certain blaze design in such a way that the refractive power of the zone varies across said zone; then the refractive power of this zone is an average power.
In conventional multifocal diffractive lenses with optical steps between adjacent zones, none of the various diffractive powers of the lens are equal to the refractive power of the zones. In particular, this is true also for the zeroth diffractive power of a diffractive lens, in apparent contradiction to the terminology used by some authors who call this zeroth diffractive power the xe2x80x9crefractivexe2x80x9d power of a diffractive lens (see e.g. Freeman, U.S. Pat. Nos. 4,537,697 and 4,642,112). But even if the average optical path lengths of light rays between an object point and its conjugated image point through any two zones are equalxe2x80x94as is the case in the zeroth order diffractive powerxe2x80x94this power is not a refractive power, since it cannot be calculated or derived on the basis of the refraction law for light rays, i.e. without wave considerations.
There are two principal designs of diffractive lenses. In the first design, the optical path length difference t between the first and second zone is equal to the path length difference between the second and the third zone, etc. Embodiments of such diffractive lenses usually exhibit a saw tooth profile on one of the surfaces of a lens made from a material of some given refractive index. This saw tooth profile can be embedded in a material of different refractive index in order to obtain e.g. smooth outer surfaces of the bulk lens. FIG. 1 is a schematic sketch of the center portion of a diffractive lens according to such a design of the prior art. When applied to a contact lens, the saw tooth profile is usually present on the back surface of the lens in order to control the phase relations of such lenses. The saw tooth profile 4 is completely embedded in the tear layer 1 between the cornea 2 and the diffractive lens 3; thus definite conditions for the phase relations of the diffractive lens are guaranteed. The lens 3 has to be made, of course, from a material whose index of refraction is different from the refractive index of the tear fluid. Although in such a design comfort may be compromised by the presence of circular grooves on the backside, such a design is presently the only one which has obtained practical importance in ophthalmic optics. Putting the saw tooth profile on the front surface results in smaller acceptable machining tolerances, since abs(nLxe2x88x921) is usually larger than abs(nLxe2x88x92nT), wherein nL is the refractive index of the lens and nT the index of the tear fluid. Also, a tear layer on front grooves of a diffractive lens can compromise the optics of such a lens, since the tear layer thickness will most likely be non-uniform.
In the second principal design of prior art diffractive lenses, the optical path length differences between the first and second zone is +t; between the second and third zone is xe2x88x92t; between the third and forth zone is +t; etc. FIG. 2 is a schematic sketch of the central portion of a contact lens according to this prior art design, in comparison with FIG. 1. Although it would seem that such a lens rests more comfortably on the eye, contact lenses of this design have not gained major practical importance. The reasons for this are likely to be of practical nature, since it is difficult to cut such lenses or molds for such lenses. More specifically, two adjacent comers 5 and 6 of any zone would have to be cut by diamond tools of different orientation, since the groove cross-section should be rectangular and not trapezoidal.
Combinations of the aforementioned designs are possible and occasionally mentioned in the patent literature.
The drawbacks of any of the presently known diffractive lenses can be summarized as follows:
1) Diffractive lenses or molds for diffractive lenses are difficult to machine since such lenses require exact grooves on at least one surface with groove depths in the order of microns only.
2) Due to machining imperfectionsxe2x80x94caused by the non-zero diamond tool radiusxe2x80x94the theoretical profile cannot be machined to exactness. As a consequence, practical embodiments of such lenses exhibit a sizeable portion of non-optical surfaces. FIG. 3 compares the ideal theoretical zone profile with its corresponding practical embodiment for a lens according to the first prior art design. In FIG. 4 the comparison is for a lens made according to the second prior art design. Non-optical surfaces result in stray light, loss of in-focus light intensity and reduced contrast.
3) In ophthalmic lenses, grooves on the surface give rise to accumulation of debris, which compromises optical performance of the lens.
4) The flanks of the groovesxe2x80x94labeled 7 in FIG. 3 and 8, 9 in FIG. 4xe2x80x94which are essentially parallel or slightly inclined to the lens axis tend to reflect incoming light. Such reflected light is lost in the foci and leads to the experience of halos by the lens user.
5) Diffractive lensesxe2x80x94even if manufactured to near perfectionxe2x80x94exhibit relatively high longitudinal chromatic aberration in at least one of the diffractive powers. This holds true in particular for lenses according to the first prior art design discussed above. Although some authors describe such chromatic aberration as beneficial in ophthalmic applications, the magnitude of this chromatic aberration should be maintained within certain limits, since sizably different powers for blue and red light may compromise visual resolution in the case of multi-colored objects (e.g. color prints).
6) In diffractive lenses according to the above designs, it is difficult to provide more than two main powers. Lenses with more than two main powers require peculiar zone blaze designs which are difficult to fabricate in practice.
The principal inventors of diffractive lenses of the prior art embodiments discussed above are Cohen and Freeman. The Cohen patent family encompasses in essence the following patents: U.S. Pat. Nos. 4,210,391; 4,338,005; 4,340,283; 5,054,905; 5,056,908; 5,117,306; 5,120,120; 5,121,979; 5,121,980; 5,144,483. U.S. Pat. No. 5,056,908 discloses an ophthalmic contact lens with a phase plate and a pure refractive portion within its optic zone. Freeman""s patent family on diffractive lenses consists in essence of the following patents: U.S. Pat. Nos. 4,637,697; 4,642,112;4,655,565 and 4,641,934. Still other patents on diffractive lenses were granted to e.g., Futhey (U.S. Pat. Nos. 4,830,481; 4,936,666; 5,129,718; 5,229,797), Taboury (U.S. Pat. No. 5,104,212), Isaacson (U.S. Pat. No. 5,152,788) and Simpson (U.S. Pat. No. 5,116,111). Common to all designs of the mentioned patents is the fact that optical steps are present between adjacent zones of such diffractive lenses. As a consequence, at least one of the surfaces of such diffractive lenses has a saw-toothed like profile with geometric steps.
Diffractive multifocal lenses are also disclosed in U.S. Pat. No. 5,760,871 to Kosoburd, et al. in which the geometric surface pattern is not saw-tooth, but is constructed as a periodic geometric function such as a cosine or a xe2x80x9csuper Gaussianxe2x80x9d. The ""871 patent discloses that such diffractive geometric profiles are suitable for trifocal diffractive lenses, in which the intermediate power of the undiffracted light is accompanied by a xe2x88x921st and a +1st diffractive order power, respectively.
As an alternative to diffractive bifocal lenses, so-called refractive bi- and/or multifocals have gained some practical importance. Such prior art lenses are either of the multiple annular zone type (FIG. 5) or are so-called aspheric designs (FIG. 6). Multi-zone refractive bifocals exhibit e.g., a far power in the odd zones 10 and a near power in the even zones 11 of the lens. The zones must not be Fresnel zones, since then such lenses would degenerate into multifocal diffractive lenses.
Refractive bi-and multifocal contact lenses are described in, e.g. M. Ruben and M. Guillon, ed. xe2x80x9cContact Lens Practicexe2x80x9d, Chapman and Hall Medical, London 1994, pp. 771. Typical embodiments were designed e.g. by Wesley (U.S. Pat. No. 3,794,414), de Carle (U.S. Pat. No. 4,704,016), Greendahl (U.S. Pat. No. 4,795,462), Marie (U.S. Pat. No. 5,106,180), Neefe (U.S. Pat. No. 3,560,598), Kelman (U.S. Pat. No. 4,728,182) and Tsuetaki (U.S. Pat. No. 3,431,327).
Diffraction analyses of multi-zone refractive multifocals teach that the optical path length between an object point and its conjugated image point through a zone of given refractive power is different from the optical path length between the same object and image points, if the light is refracted by another zone of identical refractive power. As a consequence, the associated light waves are not in phase in the image point, which results in reduced intensity and contrast.
In aspheric designs, the (theoretical) refractive power changes continuously from the center portion to the annular rim portion of a lens in order to focus object points at different distances into one and the same image point. The implication in such designs is that light rays through any particular position of the lens would be refracted in total independence of the other light rays through the lens. Trivially, this is not the case, and diffraction analyses explain the sometimes unexpected and usually poor performance of such lenses. In FIG. 6, an aspheric multifocal lens exhibits e.g. a spherical back surface 13 and an aspheric front surface 12. According to considerations of purely refractive opticsxe2x80x94an approximation which does not hold truexe2x80x94a light ray 16 close to the lens axis would be directed into the focus 17 and a ray 14 into focus 15. Rays between the positions of rays 14 and 16 would be directed into focal points between 15 and 17.
The principal deficiency of all so-called refractive bi- and/or multifocal lensesxe2x80x94be it of the multi-zone or of the aspheric designsxe2x80x94can therefore be summarized as follows:
1) In the design of so-called refractive multifocal lenses, diffraction or light interference effects are not taken into consideration. As a consequence, waves from different portions of such lenses exhibit uncontrolled phase differences in any (multi-zone lenses) or all (aspheric lenses) of the xe2x80x9crefractivexe2x80x9d foci of such lenses. Uncontrolled out-of-phase vector addition of light waves leads to reduced intensity and reduced contrast in the design powers of such lenses or the absence of such design powers.
2) Since different powers are within different aperture stops (i.e. pupils), the predominant power and/or the intensity distribution between the various powers of such lenses are dependent on pupil size. For example, in aspheric designs according to FIG. 6, distance visual acuity is very poor with small pupil size (bright light) conditions.
Finally it is mentioned that designs are also known in which purely refractive powers are combined with purely diffractive powers. A contact lens may e.g. have a purely diffractive bifocal central zone which is surrounded by a purely refractive monofocal zone. Also designs are known in which a so called xe2x80x9crefractive channelxe2x80x9d(see U.S. Pat. No. 5,056,908 to Cohen), i.e., a purely refractive part is present within an otherwise diffractive lens. As will be appreciated, such lenses are also pupil size dependent, since the refractive portion of the lens is monofocal and the diffractive part is bifocal.
It is an object of the present invention to provide a multifocal lens including a plurality of annular zones, and each annular zone is divided into at least two annular sub-zones such that the refractive powers within the sub-zones exhibit at least two diffractive powers and at least one of the diffractive powers substantially coincides with the average refractive power of each annular zone.
It is an object of the present invention to provide a multifocal lens which exhibits neither optical nor geometrical steps on any of its surfaces.
It is a further object of the present invention to provide a multifocal lens which essentially does not exhibit non-optical surfaces.
It is another object of the present invention to provide a multifocal lens which does not have the tendency to accumulate debris or dirt in any portion of the lens.
It is a further object of the present invention to provide a multifocal lens which does not reflect light into any non-focal position.
It is still another object of the present invention to provide a multifocal lens which has no longitudinal chromatic aberration in at least one power and only moderate longitudinal chromatic aberration in any of its other powers.
It is a particular object of the present invention to provide a multifocal lens which exhibits at least two different powers in which any of the powers can be attributed a relative intensity in independence of the relative intensity attributed to the other power.
It is another particular object of the present invention to provide a multifocal lens which exhibits at least three different powers in which any of said three powers can be attributed a relative intensity in independence of the relative intensities attributed to the other two of said three powers.
It is a still another particular object of the present invention to provide a multifocal lens in which the average phases of partial waves from different lens portions are controlled in the various powers or foci.
In accordance with one form of the present invention, a lens is divided into annular zones. Each annular zone is subdivided into at least two sub-zones. A refractive power is given to one of the sub-zones of every annular zone. The assembly or combination of the sub-zones with refractive power forms a diffractive lens having two main diffractive powers if an optical path length difference is introduced between the sub-zones. The remaining sub-zones of the annular zones are now given refractive powers such that the average refractive power of every entire annular zone is equal to one of said diffractive powers. As will be shown, one of the two principal diffractive powers of the lens is then identical with the average refractive zone power. Preferably, no optical steps are provided between any annular zones or between the sub-zones of any of said annular zones.
In another form of the present invention, a lens is divided into annular zones. Each annular zone is subdivided into at least two sub-zones. A refractive power is given to one of the sub-zones in all odd numbered annular zones, and a refractive power is given to one of the sub-zones in all even numbered annular zones. Then the assembly or combination of said sub-zones with refractive power forms a diffractive lens with two main diffractive powers if optical path lengths differences are introduced between the odd and the even sub-zones. The remaining sub-zones of all odd and even annular zones are given refractive powers such that the average refractive powers of the odd annular zones are equal to one of the main diffractive powers, and the sub-zones of all even annular zones are given refractive powers such that the average refractive powers of the even annular zones are equal to the other of the main diffractive powers. As a consequence, both of the main diffractive powers of the lens are at the same time refractive powers of the lens. Preferably, no optical steps are provided between any zones or sub-zones of the lens.
In a still further form of the invention, a multifocal lens exhibiting at least two principal lens powers D1 and D2 includes a plurality of annular zones. Preferably, there are no geometric or optical steps between the annular zones. Each annular zone j is subdivided into at least two annular sub-zones, i.e., a main sub-zone and a phase sub-zone. Preferably, there are no geometric or optical steps between the main sub-zones and the phase sub-zones. Each main sub-zone of the annular zone j exhibits a refractive power Dj,G and each phase sub-zone of the annular zone j exhibits a refractive power Dj,S. Each annular zone j exhibits an average refractive power D1,j=Dj,G((1xe2x88x92pj)+Dj,Sxc3x97pj, wherein pj is the fraction of the phase sub-zone of the entire annular zone j. The average refractive power D1j includes a first principal zone power. Additionally, each annular zone j exhibits an inner bonding radius rjxe2x88x921 and an outer bonding radius rj, such that the bonding radii provides a power difference xcex94Dj=2xcex/(rj2xe2x88x92rj-2) of the annular zone j, wherein xcex is a design wavelength. A second principal zone power D2j is given by D2j=D1jxc2x1xcex94Dj, such that the principal lens power D1 is the average of the principal zones powers D1j, and the principal lens power D2 is the average of the principal zone powers D2j.
In a preferred embodiment, the annular zones of a lens according to the present invention exhibit equal areas. The sub-zones of a lens according to the present invention may exhibit different areas in different annular zones. The design parameters for the lens are equally applicable to manufacturing multifocal mirrors.
Still other forms of the present inventions will be discussed in the following detailed description of preferred embodiments.